WASHINGTON (Reuters) - U.S. health officials are due to release new estimates of deaths from swine flu on Thursday, but the numbers will be just that -- a rough estimate.

The U.S. Centers for Disease Control and Prevention and the World Health Organization stopped trying to count actual cases months ago, once it became clear that H1N1 was a pandemic that would infect millions.

There are nowhere near enough diagnostic tests to give to everyone with flu-like symptoms to see if they really have swine flu, and autopsies have shown that some people who have died had H1N1 and no one even knew it.

So the death figures will be based on models, calculated by looking intensively at small groups of people, gathering data on overall reports of sickness and death, and reconciling the two.

This is also what happens every year with seasonal influenza, which WHO says kills 250,000 to 500,000 people a year globally and which CDC says kills 36,000 Americans in an average year.

Will this pandemic kill even more?

Not necessarily. Seasonal flu attacks about 20 percent of the population in an average year but it is the elderly who are the most likely to die. These patients often have other conditions and a flu infection can be the last straw that kills them.

H1N1 is hitting a younger population -- adults in their 20s and 30s and children. The latest counts from CDC showed that 1,000 have died so far, including 129 children.

The global count is more than 6,000, according to WHO.

It is possible that these younger patients may be more likely to survive their bout of flu, even if they have chronic conditions.

Doctors are comparing information about who is the most likely to die.

MEXICAN STUDY

A study in The Lancet medical journal released late on Wednesday shows that in Mexico, where the new flu appears to have spread first last March, young people were the most likely to be infected but elderly were most likely to die.

This study matches one published last week in the Journal of the American Medical Association that showed 11 percent of Californians hospitalized for H1N1 died, but among people 50 and older, 18 to 20 percent who went to a hospital died.

A CDC estimate released last month suggested that up to 20,000 people were hospitalized with H1N1 through July and that 6 percent of hospitalized patients died, or about 1,300 people.

Seasonal flu has a death rate of less than 0.1 percent. The worst pandemic, such as the influenza pandemic of 1918, had a mortality rate of 2 percent or more.

The Mexican study also found that infants and people aged 39 years and under were the most likely to get infected, but that far fewer than 1 percent of these patients died.

Ten percent of patients over 70 who were treated in clinics died, they found.

They found that 4.5 percent of patients aged 50 to 59 died, but just 2.7 percent of those in their 40s and 2 percent of patients in their 30s.

These were all people who attended clinics that were part of the Mexican Institute for Social Security network, the Institute's Victor Borja-Aburto and colleagues reported, so milder cases for which patients did not seek treatment were not included in the analysis.

Mexicans who had been vaccinated for seasonal influenza had a 35 percent lower risk of getting H1N1, even though the seasonal flu vaccine offers no protection against the new virus.

Every day of delay in hospital admission after the fourth day of illness raised the risk of death by almost 20 percent, Borja-Aburto's team found.

The study shows hard hard it is to get a grip on flu deaths, as doctors cannot assess or count people who do not show up for treatment.

And numbers take months to collect. The latest Mexican data includes cases from April to July.